Schedule task to derive code mappings when a new group is found

[armenzg]

We currently schedule the task to derive code mappings too often. I want to reduce this unnecessary load.

We currently schedule tasks in post_process (700M+ events in the last day), however, I want to move it to when we need to find a group for the event (in the last 24 hours about 0.5% of events create a group).

[*] Data source "Seer Grouping" Datadog dashboard